Berg, Sven

Abstract [en]

The use of diffusers is very common in all sorts of applications. Its main function is to recover the kinetic energy of the flow. The diffuser does this by expanding the flow and thereby regaining pressure energy. The purpose of the master thesis was to find out if it is possible to prevent separation and thereby reduce the energy losses in a three-dimensional flow. The investigation concerned the use of jet injection to prevent the separation. The experimental set-up was a rectangular closed channel with an inlet area of 0.05x0.1 m2 and outlet area of 0.1x0.1 m2. This model created a separation by expanding the flow with a backward-facing ramp. Visualisations of the flow were made with laser-sheet and fluorescent ink. Velocity measurements were made with the LDA technique while U-tube manometers were used to measure the pressure. The energy losses were calculated to check the effect of the jet injection on the flow quantitatively. Results indicate that flow-separation control can be accomplished, with the control being a function of the mass flow rate. Visualisation shows that the jet injection prevents separation for jet mass flow of 1.25-2.25% of the total flow rate. The energy measurements show that the reduction in energy losses provided by the jets is less than the energy required to drive the jet flow.